As a drain recovery system of this type, an open drain recovery system as described in Patent Literature 1 in which drain is utilized by recovering the drain in an air-open-type drain tank and supplying the drain to a steam boiler is known. According to this open drain recovery system, an effective drain recovery rate is up to 40% to 50% at a maximum, as a large amount of flush steam is generated when high-temperature and high-pressure drain flows into the drain tank. Therefore, as in Patent Literature 1, various attempts have been made in order to recover the flush steam. However, an effective drain recovery rate of an entire system is limited despite such attempts. Here, the effective drain recovery rate is defined as “(returned drain amount−amount of flush steam released into air)/returned drain amount”.
As a method that fundamentally solves this problem, a closed drain recovery system as described in Patent Literature 2 in which drain is utilized by recovering the drain in a closed-type drain tank and supplying the drain to a steam boiler has been known. This closed drain recovery system can improve the effective drain recovery rate, as it prevents flush steam generated in the drain tank from being released into air without recovering heat. Further an improvement of the recovery rate is expected by increasing a content of the drain tank against the returned drain amount.